The invention relates to acoustic liners for magnetic field gradient coils, in particular those used for magnetic resonance imaging (MRI).
In a magnetic resonance imaging (MRI) system, a gradient coil is employed to produce a linear magnetic field gradient, defined with respect to 3 orthogonal directions (X, Y and Z). The gradient coil is often a cylindrical structure placed in the center of the bore of a superconducting magnet where a strong and stable magnetic field exists. The gradient coil generally consists of a set of copper conductors arranged in a pattern around the surface of the cylindrical structure. The pattern is calculated to produce the linear field gradient, and methods of calculation and construction of such gradients are well described in the literature. Most gradient coils are of the xe2x80x98active screenedxe2x80x99 type whereby a primary and secondary set of conductors are provided spaced radially apart and connected in series opposition so as to cancel any external field. This arrangement is used in order to reduce eddy currents induced in conducting materials, for example the cryogenics of a superconducting magnet, located outside the structure.
The gradient coils do not normally provide a constant static field gradient. In operation, a pulse of current is applied to the gradient coils to produce the required field gradients over a short period of time. There are a variety of different pulse sequences commonly used. Most current pulses have a short xe2x80x98rise-timexe2x80x99 of the order of 1 millisecond and are repeated at regular intervals during the acquisition of an image.
A common problem with gradient coils is the acoustic noise which is generated during operation.
When a current pulse is applied to the gradient coils in the presence of a magnetic field, a Lorentz force acts on the conductors, causing them to move. Since the pulses are applied at a relatively high frequency, the result is that the gradient coil structure vibrates, creating acoustic noise. As most MRI systems are designed such that there is a patient in the form of a person or animal located inside the magnet and gradient coils, this acoustic noise can be uncomfortable for the patient. Indeed, as magnetic fields become larger and higher gradient strengths are used the acoustic noise can be so intense as to constitute a danger to the patient""s hearing, and measures must be taken to protect the patient from it.
A need therefore exists to reduce or eliminate the acoustic noise produced by a gradient coil.
Previous attempts at reducing the noise have tended to concentrate on making the structure of the gradient coils as stiff and as heavy as possible to reduce the amplitude of vibration, or to provide a layer of xe2x80x98soundproofingxe2x80x99 material within the gradient in order to absorb the mechanical energy.
U.S. Pat. No. 5,764,059 discloses a means of reducing acoustic noise by encasing closed loop conductive pathways in a potting material chosen to maximise the velocity of sound within it. The Lorentz forces acting on different parts of each loop cancel each other so that the total resultant force on the loop is zero. The vibration of opposite sides of each loop with respect to each other is prevented by means of encasing the loop in rigid material.
The present invention provides an acoustic liner for use with a magnetic field gradient coil in a magnetic resonance imaging (MRI) system, the liner comprising
an acoustic sheet, adapted to be fitted to the gradient coil in such a way that the inner surface of the sheet is movable relative to the gradient coil, and
an acoustic conducting path for carrying an electrical current provided within or connected to the sheet,
arranged such that in use a current pulse can be applied to the gradient coil and a current pulse can be applied to the acoustic conducting path in synchronism therewith, wherein
the acoustic conducting path is arranged such that Lorentz forces on the acoustic conducting path in the magnetic field produced by the MRI system cause the acoustic conducting path to move relative to the gradient coil in such a way that the inner surface of said sheet remains substantially stationary.
The acoustic conducting path is preferably in the form of a coating which is so thin that it is flexible, to which can be applied a current pulse in synchronisation with the current pulse applied to the gradient coil. The compressible material may be rubber foam.
When a current pulse is applied to the gradient coils, a corresponding pulse is applied to the acoustic conducting path and there will be a Lorentz force on the acoustic conducting path, just as there will be on the gradient coils. This will cause it to move and compress (or expand) the flexible compressible material. If the phase and amplitude of the current pulse are chosen to cause the inner skin to move an equal and opposite amount to the original gradient inner surface then the net movement of the surface of the sheet of compressible material will be zero. As this is the surface in contact with the air, little or no sound will be generated.
The gradient coil is very stiff whilst the sheet is very compressible. The force required to cause movement of the compressible sheet is therefore much less than the force required to cause the same movement of the gradient coil. Therefore that the current required to move the acoustic coils is much less than the current used to drive the gradient coil.
The acoustic conducting path will obviously have an effect on the magnetic field gradient generated. Due to the fact that the current through it is much less than the current through the gradient coils this will not always be significant, but it is important to minimize this effect to prevent any interference with the imaging process.
One way of achieving this is to provide that the liner flirter comprises an additional acoustic conducting path for carrying an electrical current located on the surface of the sheet of flexible, compressible material adapted to be fitted to the magnetic field gradient coil. Preferably the acoustic conducting path and the additional acoustic conducting path are connected in series opposition.
The acoustic conducting path and the additional acoustic conducting path may comprise identical conducting paths separated by the sheet of compressible material.
As the two conducting paths are wired in opposition the current flowing on one side of the compressible material will be in the opposite direction to the current flowing on the other at all positions and the net magnetic field produced will be small. A refinement to this is to calculate the two conducting paths either side of the compressible material to be slightly different such that the net magnetic field produced by the pair at the center is further reduced.
Preferably the acoustic conducting path and the additional acoustic conducting path are arranged such that the coupling between the gradient coil and the sum of the acoustic conducting path and the additional acoustic conducting path is substantially zero. This will ensure that little or no current or voltage will be induced by the gradient coil across the pair of conducting paths.
The acoustic conducting path and additional acoustic conducting path may be arranged such that there is a mutual inductance between the acoustic conducting paths and the gradient coil, whereby the resistance, inductance and coupling constant to the gradient coil are such that the induced current and corresponding decay characteristic in the acoustic conducting paths achieve at least in part the desired acoustic cancellation.
An alternative method of providing acoustic conducting paths which will not interfere with the imaging process is to arrange for a single pattern on the inner skin of the liner, wherein the pattern itself is arranged to produce a linear field gradient whilst still matching as closely as possible the vibration pattern of the gradient coil. If this field is small compared to the field produced by the gradient then it can be accommodated without detriment to the imaging process by xe2x80x98signal conditioningxe2x80x99 of the gradient drive such that the gradient field and the field produced by the acoustic conducting path add to produce the resultant field originally required.
An alternative embodiment of the invention is to use a rigid tube as the acoustic liner, but provide active support means at a plurality of points around its surface between the gradient coil and acoustic liner it e.g. piezoelectric position transducers. During operation of the gradient coils an appropriate signal is applied to the active support means in anti-phase to the vibration of the gradient coil at the position of each support means such that the motion of the acoustic liner is cancelled.
According to a second aspect, the invention provides a means for generating a magnetic field gradient, comprising
a field gradient coil,
an acoustic sheet, fitted to the gradient coil in such a way that the inner surface of the sheet is movable relative to the gradient coil
an acoustic conducting path for carrying an electrical current provided within or connected to the sheet,
means for providing a current pulse to the field gradient coil and a current pulse to the acoustic conducting path in synchronism, wherein
the acoustic conducting path and the means for providing a current pulse thereto are arranged such that, in use, Lorentz forces on the acoustic conducting path cause the acoustic conducting path to move relative to the gradient coil in such a way that the inner surface of said sheet remains substantially stationary.
In a further embodiment of the invention, a similar acoustic liner is applied to the ends of the magnet structure. The gradient coil is mounted to the magnet and its vibration will be transmitted to the magnet itself which will produce acoustic noise. The acoustic liner at the end would take the form of a substantially flat xe2x80x98pancakexe2x80x99 fixed to each end and would prevent acoustic noise being transmitted into the room occupied by the magnet.